Technology for improving STP protocols in ethernet networks supporting VLANs

ABSTRACT

A method for utilizing a Spanning Tree Protocol (STP) in an Ethernet network wherein a number of VLANs are defined and a plurality of Ethernet switching nodes (A, B, C D E, F,G H) are interconnected via their ports (A 1 , . . . ,H 4 ) so that each of the ports is initially assigned to one or more VLANs, and each of the VLANs is intended to enable traffic between two or more edge ports (D 3 , B 3 ). The method comprises a so-called pruning procedure initiated upon establishing a new STP topology ( 30 ) related to one or more VLANs. This procedure concerns a broadcast domain of at least one of the VLANs and is aimed at obtaining a sub-tree for each pruned VLAN in the new STP topology. The sub-tree is usually shrunk, bound by the edge ports assigned to the corresponding pruned VLAN, and thus eliminates broadcast traffic of the pruned VLAN to any Ethernet switch extending beyond the obtained sub-tree.

FIELD OF THE INVENTION

The technology relates to Ethernet networks supporting VLANs andutilizing a Spanning Tree Protocol in its various versions.

BACKGROUND OF THE INVENTION

As part of the background description, the following terms are to bebriefly explained:

STP—Spanning Tree Protocol (including its various standard variationssuch as STP—IEEE 802.1D, RSTP-802.1w, or MSTP-802.1s), is standard forprotection and loop prevention in Ethernet communication networks. TheSTP recalculates the active topology in case of faults orreconfiguration. The STP also determines the active topology (STPnetwork topology) to carry traffic upon initialization. According toSTP, any network can logically be represented in the form of one or moreinterconnected trees—STP trees. An Ethernet network may be covered by asingle STP tree; one STP tree may be provided per one VLAN; one STP treemay cover a group of VLANs.

VLAN—Virtual Local Area Network forming a logical partitioning of anEthernet network (e.g. as defined per IEEE 802.Q) and defining abroadcast domain, within the Ethernet network;

GVRP—Generic VLAN Registration Protocol—Protocol for dynamicallyregistering/provisioning or deregistering VLANs on a per VLAN basis.

Switching node—(or Ethernet switch or bridge) a network node having anumber of ports for receiving and transmitting data frames.

Forwarding port—a port of a switching node being presently active,according to a presently actual STP topology, to receive data frames andforward them to one or more other ports of this or another node.

Ports assigned to a particular VLAN—ports of a switching node which,according to a selected service, are predestined for transmitting datapackets to/from the particular VLAN.

Forwarding port—a port of a switching node

Edge port—a port that is on the boundary of an STP/VLAN domain; the edgeport is considered a permanently forwarding port.

FDB—Filtering database—a standard database of MAC addresses in anEthernet switching node, which is self-learned i.e., gradually formedduring operation and based on registering source addresses from whichdata frames arrive to particular ports, thereby FDB “learns” the networkfor facilitating further switching decisions.

PDU—(Protocol Data Unit) a management message in any of the concernedprotocols such as STP, GVRP, or the proposed below technique/protocol.

Spanning Tree Protocol (STP) is widely used for loop prevention andprotection in Ethernet switched networks. Ethernet switches (nodes)forming ring or mesh networks might be connected to Local Area Networks(LANs) or end stations. A LAN may also comprise a number of Ethernetswitches, where some support terminal/access ports (i.e. those directlyconnected to a LAN end-station or to those on the boundary of one STPdomain to another). For increasing the scalability of Ethernet switchednetworks, also utilized are so-called Virtual Local Area Networks(VLANs). A number of VLANs are usually defined in the network.

VLANs may overlap one another, and normally, each particular VLAN mustspan each Ethernet switch that may be encountered in a path from oneterminal to another, in any possible topology that may be imposed by theSTP. Such a configuration of VLANs will be further called initial VLANsconfiguration. In many cases it can be that the initial configurationrequires that all VLANs are provisioned on all ports throughout thenetwork that are not edge ports. VLANs can be configured on the Ethernetnetwork by GVRP or a management interface such as SNMP.

U.S. Pat. No. 6,515,969 relates to a method and apparatus fordisseminating Virtual Local Area Network (VLAN) membership informationacross computer networks defining multiple spanning trees, which ishereby incorporated by reference. An intermediate network deviceincludes a plurality of ports and a plurality of spanning tree engineseach associated with one or more VLAN designations defined within thenetwork. The spanning tree engines transition the ports among aplurality of port states, including a forwarding state and a blockedstate. For each port, a separate Generic Attribute Registration Protocol(GARP) participant is also established and each GARP participantincludes a multiple spanning tree (MST) GARP VLAN Registration Protocol(MST-GVRP) application component and an associated GARP InformationDeclaration (GID) component. The MST-GVRP application componentscooperate to define a plurality of GARP Information Propagation (GIP)contexts each of which is associated with a spanning tree engine andthus its one or more VLAN designations. The technology described in theU.S. Pat. No. 6,515,969 relates to establishing the network topologywhenever required, no optimization or special provisions for topologychange and transition periods are mentioned. Also, the patent does notdescribe any solution for reducing broadcast traffic within a VLAN.

For addressing a particular data packet (frame) from a source node to adestination node, protocol for Ethernet networks supporting VLANs (IEEE802.Q) requires that the data frame carry indication of a specific VLANto which both the source and the destination node(s) belongs. Accordingto the basic approach, Ethernet switches, passing there-through a datapacket which is not addressed to a known MAC address but carriesindication of a specific VLAN, performs “flooding” of all its outputports assigned to that VLAN. Thus, all the relevant neighboring nodesare enabled to continue checking and forwarding that packet for thespecified VLAN and, if indicated and known within the switch, to thespecified destination MAC address.

As mentioned, any Ethernet switch supports the Filtering Database (FDB)which is gradually formed by recording the source address indicated in adata packet (i.e. Ethernet frame) when the frame is received by one ofthe ports. A frame arriving to the switch and destined to a specificunicast MAC address that exists in the FDB, is not broadcasted to alloutput ports of the node, but is switched to a particular port aspresently known to the switch with respect to that address. Therefore,the FDB allows reducing excessive flooding of traffic in the network.

It should be noted that an STP protocol, when applied to Ethernetswitched network, creates a so-called “logical cut” in each cycle/ring,to impose an active tree topology. Consequently, all FDB informationmust take into account the given STP tree structure. If a fault occursin any span of the structure, the STP protocol urgently reacts to thatand recalculates the tree structure(s). The recalculation of theSpanning Tree results in a different network topology which, of course,will dictate other switched paths between those source nodes anddestination nodes which are usually in communication. Significantportions of the FDBs supported by the switches associated with thechanged STP tree topology are deleted as soon as the topology ischanged. The changes in the STP topology cannot be processed immediatelyso, at the beginning, a huge amount of traffic is broadcasted or floodedat the switching nodes of the network (since a significant part of theFDB or the complete information previously recorded by FDB is deletedand new information is not yet collected).

SUMMARY OF THE INVENTION

It is therefore the object of the present invention to reduce unneededtraffic load (broadcasted traffic, flooded traffic) in an Ethernetnetwork during and after transition (learning) periods caused byinitialization or change of an STP network topology.

It should be noted that an STP topology change may be triggered byestablishing the initial network topology, by one or more faults in thenetwork, or by any “topology affecting” modification of STP parametersperformed by an operator (both logical and physical modifications).

The object can be achieved by providing a method for utilizing andenhancing an STP protocol to be supported by an Ethernet network whereina number of VLANs are defined and a plurality of Ethernet switchingnodes are interconnected via their ports so that each of the ports isinitially assigned to one or more VLANs, and each of the VLANs isintended to enable traffic between two or more access/edge ports;

the method, upon establishing a new STP topology related to one or moreVLANs, initiates pruning the broadcast domain of at least one of saidone or more VLANs to obtain at least one VLAN-related sub-tree of thenew STP topology, the VLAN-related sub-tree being bound by access/edgeports assigned to its corresponding VLAN,

the method thereby substantially reducing broadcasted and floodedtraffic in the network.

More exactly, as a result of the proposed method, the broadcast trafficcan only be detected at those Ethernet switch nodes which belong to theVLAN-related sub-tree obtained owing to the pruning procedure. Nobroadcast traffic of the pruned VLAN will be detected beyond thesub-tree. In other words, the method eliminates broadcast traffic ofeach said pruned VLAN beyond its obtained sub-tree.

The sub-tree is usually the only possible path or connection between theedge ports in the new STP topology (being also a tree structure).

In the simplest case, the sub-tree is a path defined by two edge portsforming its end points.

The object of the invention can be achieved if the following step isperformed upon establishing the new STP topology in the network:

transmitting at least one de-activation message from at least oneEthernet switching node participating in the new STP topology to aneighbor Ethernet switching node also participating in the new STPtopology, the message indicating: one or more VLANs de-activated withrespect to a port transmitting said message, so as to allow said one ormore VLANs be de-activated with respect to a port receiving saidmessage,

said at least one message allowing cancellation of redundant portions ofthe new STP network topology with respect to said one or more VLANs bypruning the broadcast domains of said one or more VLANs, therebyoptimizing the STP network topology and reducing useless data traffic ofsaid VLANs in the network.

More particularly, the method comprises the following steps to beperformed at each particular Ethernet switching node participating inthe new STP topology and with respect to each of said VLANs:

-   -   a) ensuring that ports of the Ethernet switching node        participating in the new STP topology are assigned to said one        or more VLANs according to an initial VLANs configuration;    -   b) counting forwarding ports assigned to a particular VLAN,        being active according to the new STP topology;    -   c) if a port assigned to a particular VLAN is a single        forwarding port of said particular VLAN at said node,        de-activating said port with respect to said VLAN, thereby        pruning said VLAN at said port and said node;    -   d) generating said de-activation message from each said single        forwarding (de-activated) port of said node and performing said        step of transmitting said de-activation message to a neighbor        Ethernet switching node in the new STP topology;    -   e) de-activating the port of the neighbor Ethernet switch, that        has received said de-activation message, with respect to said        one or more VLANs indicated in the message;    -   f) repeating the method from step (b) at said neighbor node.

The sub-step (a) means that the initial VLAN configuration is guaranteedby default for the initial establishing of the network, but should berestored in case a new STP topology change has taken place after thepruning had been performed to a previous STP topology. Owing to that,the performed pruning does not introduce any cuts that would affecttraffic once a new topology occurs.

As can be seen, the method reduces, one by one, so-called dead-end portsand nodes of the new STP topology with respect to one or more VLANs,thereby pruning VLANs in the Ethernet network to eliminate excessivedata traffic in the network upon establishing the new STP topology.

The method is also applicable to the case where the new STP topology isthe initial STP topology in the Ethernet network. The method will thusallow using of any non-optimized version of the initial network topologydefinition; the proposed VLAN pruning protocol will perform the networktopology optimization with respect to VLANs configured by any suitablemeans, such as GVRP or a management interface such s SNMP.

The counting step relies on the consideration that edge ports arepermanently forwarding ports.

Further preferably, said de-activation message is a PDU (protocol DataUnit) message utilized in Ethernet frame networks. The proposed PDU isprovided with indication of one or more particular VLANs which arepruned at the port (with respect to which said port is temporaryde-activated—i.e., until the next topology change).

In the simplest case, the de-activation (PDU) message comprisesindication of a single particular VLAN de-activated (pruned) at aparticular port, so that each additional VLAN, if de-activated at thesame port, will be indicated in an additional separate PDU generated bythe node on behalf of the single forwarding port.

However, the proposed protocol may be capable of generating a combinedde-activation message listing all VLANs previously assigned at aparticular port and de-activated (pruned) at the port owing to the stepsb, c, d, e (i.e., all VLANs for which this port is a “single forwardingport”). Such combined messages create less traffic in the network,though require more time for generating and processing at any furtherswitching node; it is understood since, for creating a combined message,complete “pruning” information on all VLANs and all ports in the nodeshould be collected.

In general, the method can be started at a switching node upon receivinga Topology Change Notification (TCN) (signaling that, say, a present STPtopology is being changed to the new STP topology, or a new topology isjust initially created), and after expiring a predetermined (standardSTP) topology change timer.

TCN is usually initiated across the Ethernet switched network by somefiber cut in a link between switching nodes, by other one or more faultsin a link or in a switching node, by configuration changes and/orinitialization.

In practice, upon receiving the TCN and expiring the timer, the processof pruning starts at so-called “leaf nodes” of the new STP topology,since by definition each leaf node is assigned to some VLANs at a singleforwarding port (this port shall be the root port). At each node that isnot a leaf node, the method will actually be started after receiving atleast one de-activation message.

The pruning of a VLAN at a particular node prevents the node fromreceiving and forwarding data packets (frames) associated with saidVLAN, and thus the VLANs pruning performed on the whole network withrespect to all ports and LANs of a new topology drastically reduces thevolume of unneeded traffic, mostly—the volume of uselessly broadcastedtraffic and the traffic of the Ethernet frames flooding all ports of aswitching node during the learning process.

It should be emphasized that the proposed technique not only allowsquick reducing of useless flooding during the learning process, but alsoenables reducing the transmission of broadcast packets throughout aparticular VLAN after the learning period, which result cannot beachieved by any of the known protocols.

The proposed method of improving the STP protocol actually constitutesan additional (supplementary) protocol which can be used to extend astandard STP protocol supported by the Ethernet network. Alternativelyand preferably, it may form an integral part or an optional feature of astandard STP protocol.

The supplementary protocol may form part of a software product beingintended for an Ethernet switch; said software product being, forexample, implemented as embedded software of an Ethernet switch and runon said switch in an Ethernet network, allows performing theabove-defined method.

Yet another aspect of the present invention is an Ethernet switchprovided with the above software product (say, in the form of a suitableembedded software) and thus capable of performing the mentioned methodin an Ethernet network.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be further described and illustrated withreference to the following non-limiting drawings, in which:

FIG. 1 schematically illustrates an exemplary network and ports ofswitching nodes which may be assigned to one or more VLANs, depending ona required grouping of access ports.

FIG. 2 schematically illustrates the initial STP network topology (thelogical topology).

FIG. 3. illustrates a network topology changed by the STP protocol aftera fiber cut has been detected in one part of the network.

FIGS. 4 a, 4 b schematically illustrate how the proposed protocol forVLAN pruning works, the diagram is a portion of an STP tree reflectingthe STP topology with respect to a particular VLAN before and after thepruning takes place.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically illustrates an Ethernet network 10 comprising threeinterconnected ring networks 12, 14 and 16 including Ethernet switchingnodes A, B, C, D, E, F, G, H, I. Each switching node is shown as asquare. Ports of the nods are shown by circles and numbered 1, 2, 3 and4. Let the network 10 support a standard STP protocol. Since the STPprotocol logically represents any ring network as a broken ring (i.e.,creates a logical cut), such exemplary logical cuts are marked by dottedlines on spans AB, DE and GF. Let the network 10 define two VLANs: VLAN1for creating possible paths in the network between access edge port D3and access edge port B3, and VLAN2 for creating possible paths betweennode/port G3 and node/port H3.

Note that under normal operational policy each VLAN must be initiallyconfigured so that to be able to support connectivity betweenaccess/edge ports for each possible spanning tree active topology(Initial VLAN configuration).

Let all ports of the network nodes be assigned both to the VLAN1 (markedby black semi-circles) and VLAN2 (marked by white semi-circles), exceptfor:

ports number 3 of nodes B and D which are the edge ports of the VLAN1and marked by the whole black circle and therefore assigned only toVLAN1;

ports number 3 of nodes G and H being the edge ports of VLAN2 (marked bythe whole white circles) and therefore assigned only to VLAN2.

It should be noted that VLAN1 does not need to be configured at portsand switches associated with ring 16 since it is not encountered in thepath of B3 to D3 in any possible topology that may be imposed by STP inthis network. However, it will be shown that even in the case that VLAN1has the illustrated redundant configuration, the proposed techniqueallows its optimization.

FIG. 2 illustrates an STP tree 20 (the existing STP topology) created bythe STP protocol on the network 10, taking into account the logical cuts(dotted crosses) on the spans AB, DE and GF.

VLAN1 Ethernet packets transmitted between edges D3 and B3 willgradually find the only possible path D-C-B (black colored circles andsemicircles).

Similarly, for VLAN2 packets sent from node G to node H, the followingpath will be gradually learnt via the ports assigned to VLAN2 (whitecolored circles and semicircles): G_E_C_I_H.

It can be understood that the learning process involves a lot of uselessbroadcasting in the network, until FDBs of the switching nodes areformed.

Now, let a complete fiber cut occur between the nodes C and I (marked bya star in FIG. 1), so the initial STP topology must be changed. Let, forexample, the STP now create the logical cut of the ring network 12 atthe place of the real fiber cut, i.e., between the nodes C and I.Simultaneously, the ports B1 and A4 which were inactive in the previoustopology, are considered active, i.e. the VLANs initial configuration isrestored. The changed STP topology (tree) 30 is shown in FIG. 3. Itshould be noted, that FDBs are partially or completely deleted when thenetwork topology changes. Apparently, paths for the Ethernet packetswhich are still bi-directionally sent between node D and node B, andbetween node G and node H will now change due to the changed networktopology.

Again, the learning process will take a considerable time during whichthe Ethernet packets are uselessly broadcasted over the network.

To prevent this ineffective use of the network bandwidth, the Inventorspropose a supplementary protocol which will be illustrated with the aidof FIGS. 4 a and 4 b.

The function of the proposed protocol is to perform optimization ofVLANs in a newly established STP network topology by deactivating thoseports which do not need to serve for a given VLAN in the new (orchanged) STP topology.

Deactivating of the VLANs on ports is triggered by TCN & topology changetimer expiry at so-called leaf nodes of the STP spanning tree, since atfirst (as soon as the topology change takes place) only leaf nodes maysupport a single forwarding port assigned to any VLAN, and such port isa typical dead-end port. The protocol proceeds by checking each of theswitching nodes having at least one port assigned to a particular VLAN,to determine whether said port is a single forwarding port with respectto said particular VLAN, wherein any edge port is considered aforwarding port. If yes, the port creates and transmits a de-activationmessage to the associated port at a neighbor node. The neighbor noderepeats the procedure and, if the conclusions are the same, also createsand transmits its de-activation message.

In a similar manner, after any change in the network topology, allswitching nodes of the changed topology should check their assignedforwarding ports and exchange messages if necessary.

It should be noted, that the check procedure includes checking the portswith respect to any VLAN to which the ports may be assigned. Thus, theprocedure leads to elimination of all dead ends of the topology and thusto the maximal economy of the network bandwidth.

FIGS. 4 a and 4 b illustrate, in more detail, how the messages pass viathe network when the proposed protocol reveals “dead ends” in a changednetwork topology.

For example, FIG. 4 a illustrates the switching nodes A,B,C,D,E,F,G,H,Iof the STP network topology 30 shown in FIG. 3. Four ports of each nodeof the nodes are shown by circles. Let FIG. 4 a reflect the STP treewith respect to VLAN1, in which all ports of the nodes were initiallyassigned to VLAN1. Let the permanent ports (edge ports) be marked withthe hatched pattern, while simple forwarding ports, active in the newtopology, are double-hatched.

After a topology change timer limit expires, switches F,G,I recognizethat they only have a single forwarding point that is assigned to VLAN1.F,G and I each send a de-activation message PDU “deactivate VLAN1” totheir designated nodes (switches), i.e., E and H respectively. As partof the procedure, ports F1, G1, I1 are deactivated with respect toVLAN1. Switch E then deactivates VLAN1 on port 2 and port 4, as soon aseach PDU is received and processed from G and F respectively. Switch Ewill follow the same procedure similarly as soon as it recognizes thatE1 is a single forwarding port with respect to VLAN1. Switch Himmediately deactivates VLAN1 on port 2. Node H, in turn, since its portH1 becomes a single forwarding port, follows the same proceduresimilarly (i.e., deactivates H1 and forwards another PDU towards nodeA). Optionally, the message “deactivate VLAN . . . ” can be implementedas a single message for a particular port with respect to all VLANs thatneed to be deactivated at that port. However, the topology 30 isillustrated only with respect to VLAN1.

FIG. 4 b illustrates results of the VLAN1 pruning procedure. Due to themessages which were shown in FIG. 4 a, nodes H, I, F, G, A and E aredeactivated, i.e. neither of their ports is now assigned to VLAN1.

Switch C deactivates VLAN1 on port 2, but does not include VLAN1 in themessage to be sent to node B because VLAN1 is not deactivated on port 4.

Finally, VLAN1 is configured as active only on switches D_C_B which areneeded to connect edge ports at D and B, and only on the required ports.

In the analogous way, due to the topology change, VLAN2 will beconfigured on switches G-E-C-B-A-H (white semicircles and circles, seeFIG. 1), and de-activated (pruned) at other switches of the networkwhich were active before the topology change. (The path between edgeports G3 and H3 has changed from G-E-C-I-H to G-E-C-B-A-H).

The described procedure can be applied, as a supplementary protocol, toan Ethernet network having any degree of complexity and connectivity.

At the end of the proposed procedure, the STP tree of the newly formedconfiguration will have active, with respect to a particular VLAN, onlythe switching nodes which, in the currently active STP topology, lie inthe path connecting the terminal edge ports.

As a result, no extra traffic will flood irrelevant ports of theswitching nodes, no extra data traffic will be sent to the deactivatednodes from their neighbor nodes, and consequently, the network will befreed from excessive traffic both early in the transition period,required for FDB-re-learning, and throughout the lifetime of a given newSTP topology.

It should be noted that if, for any reason, another topology changeoccurs, the described pruning protocol should start only after resettingthe initial assignments of switches/ports of the new topology to VLANs.In other words, any new optimization of the network should start notfrom the previously optimized one, but from the initial VLANconfiguration.

It should be appreciated that other versions of the proposed techniquecould be proposed which are to be considered part of the invention asfar as being covered by the claims that follow.

1. A method for utilizing a Spanning Tree Protocol (STP) in an Ethernetnetwork wherein a number of VLANs are defined and a plurality ofEthernet switching nodes are interconnected via their ports so that eachof the ports is initially assigned to one or more VLANs, and each of theVLANs is intended to enable traffic between two or more edge ports, themethod comprises: upon establishing a new STP topology related to one ormore VLANs, initiating a pruning procedure of a broadcast domain of atleast one of said one or more VLANs to obtain a sub-tree for each prunedVLAN in the new STP topology, wherein each said sub-tree being bound bythe edge ports assigned to the corresponding pruned VLAN, therebyeliminating broadcast traffic of each said pruned VLAN to any Ethernetswitch extending beyond the obtained corresponding sub-tree; the methodcomprising the following steps to be performed at each Ethernetswitching node that is active in the new STP topology and with respectto each of said one or more VLANs: a ensuring that ports of the Ethernetswitching node that is active in the new STP topology, are assigned tosaid one or more VLANs according to an initial VLANs configuration; b)counting forwarding ports assigned to a particular VLAN, being activeaccording to the new STP topology; c) if a port assigned to a particularVLAN is a single forwarding port for said particular VLAN at said node,de-activating said port with respect to said VLAN, thereby pruning saidVLAN at said port and said node; d) generating a de-activation messagefrom each said single forwarding port of said node, the messageindicating one or more VLANs de-activated at said port, and transmittingsaid de-activation message to a neighbor Ethernet switching node in thenew STP topology; e) de-activating the port of the neighbor Ethernetswitch, that has received said de-activation message, with respect tosaid one or more VLANs indicated in the message; f) repeating steps (b)to (e) at said neighbor node.
 2. The method according to claim 1,wherein the new STP topology is the initial STP topology in the Ethernetnetwork.
 3. The method according to claim 1, wherein the new STPtopology is a changed STP topology in the Ethernet network.
 4. Themethod according to claim 1, wherein in the counting step, the edgeports are considered permanently forwarding ports.
 5. The methodaccording to claim 1, wherein said de-activation message is a PDU(Protocol Data Unit) message further provided with an indication of oneor more particular VLANs with respect to which said port isde-activated.
 6. The method according to claim 5, wherein the PDUmessage is a combined de-activation message listing all VLANsde-activated at said port.
 7. The method according to claim 1, beinginitiated at each particular Ethernet switching node upon receiving aTopology Change Notification (TCN) and after expiring a topology changetimer.
 8. An Ethernet switch provided with means for performing themethod according to claim
 1. 9. A method for utilizing a Spanning TreeProtocol (STP) in an Ethernet network wherein a number of VLANs aredefined and a plurality of Ethernet switching nodes are interconnectedvia their ports so that each of the ports is initially assigned to oneor more VLANs, and each of the VLANs is intended to enable trafficbetween two or more edge ports, the method comprising the following stepto be performed upon establishing a new STP topology in the network:transmitting at least one de-activation message from at least oneEthernet switching node that is active in the new STP topology to aneighbor Ethernet switching node also being active in the new STPtopology, indicating one or more VLANs de-activated with respect to aport transmitting said message, so as to allow that said one or moreVLANs be de-activated at a port receiving said message, wherein themethod comprises the following steps to be performed at each Ethernetswitching node that is active in the new STP topology and with respectto each of said one or more VLANs: a) ensuring that ports of theEthernet switching node that is active in the new STP topology, areassigned to said one or more VLANs according to an initial VLANsconfiguration; b) counting forwarding ports assigned to a particularVLAN, being active according to the new STP topology; c) if a portassigned to a particular VLAN is a single forwarding port for saidparticular VLAN at said node, de-activating said port with respect tosaid VLAN, thereby pruning said VLAN at said port and said node; d)generating said de-activation message from each said single forwardingport of said Ethernet switching node, the message indicating one or moreVLANs de-activated at said port, for further transmitting saidde-activation message to the neighbor Ethernet switching node in the newSTP topology; e) de-activating the port of the neighbor Ethernet switchthat has received said de-activation message, with respect to said oneor more VLANs indicated in the message; repeating steps (b) to (e) atsaid neighbor node.
 10. The method according to claim 9, wherein the newSTP topology is the initial STP topology in the Ethernet network. 11.The method according to claim 9, wherein the new STP topology is achanged STP topology in the Ethernet network.
 12. The method accordingto claim 9, wherein in the counting step, the edge ports are consideredpermanently forwarding ports.
 13. The method according to claim 9,wherein said de-activation message is a PDU (Protocol Data Unit) messagefurther provided with an indication of one or more particular VLANs withrespect to which said port is de-activated.
 14. The method according toclaim 13, wherein the PDU message is a combined de-activation messagelisting all VLANs de-activated at said port.
 15. The method according toclaim 9, being initiated at each particular Ethernet switching node uponreceiving a Topology Change Notification (TCN) and after expiring atopology change timer.
 16. An Ethernet switch provided with means forperforming the method according to claim 9.